In a mechanical system, pressurized fluid, electricity, or another source of energy may be used to impart a motive force to a load, with the motive force being capable of moving the load over a predetermined motive distance. Various rotary or linear actuator devices or actuators can direct the motive force as needed in a precise manner within the mechanical system in order to achieve a desired end result. However, variations in environmental conditions such as temperature, as well as the physical properties or characteristics of the load, can affect the overall performance or response of a mechanical system.
Therefore, it is desirable for the system behavior to be consistent over a wide range of operating conditions including variations in temperature, age of the system, small variations in the load, etc. A key aspect of, consistent performance especially for systems with thermally activated elements, such as an SMA (shape memory alloy) actuator, is the time that the system takes to complete operation. Variations in ambient temperature, heat transfer coefficients (e.g. due to surface accumulation of grime and oxides) and even changes in the effective load due to changes in system friction, all tend to change the response time of the system.
One way to mitigate the influence of the above variations on the response time of the system is to prime, or pre-heat, the actuator. In thermally-activated SMA systems this implies, heating the SMA to a temperature that is less than the actuation temperature of the SMA. This helps reduce the variation in the system's response time by reducing the time required to sensibly heat the SMA from its starting temperature to its actuation temperature. The remaining part of heating the SMA involves supplying the enthalpy to drive the phase transformation which produces actuation of the system. This is typically faster than the sensible heating part. Moreover, the enthalpy of transformation is less sensitive to the variations in the system and its ambient conditions than the sensible heat part. Therefore, minimizing the time required for sensible heating via priming ensures a more consistent system response time regardless of the variations discussed above.
One approach to estimate the priming stimulus level includes look-up tables based on expected changes in the system. However, this does not cover situations when the system or the environment changes in manner that is not covered by the look-up tables. Another approach is using environment and feedback control systems that monitor the behavior of the system using multiple sensors. However, this results in a high cost system having increased complexity.